Signal-processing apparatus utilizing variable threshold limiting means for an fm/fmmultiplex signal



B. D. I oUGHLlN 3,163,717 SIGNALPROCESSING APPARATUS UTILIZING VARIABLE THRESHOLD LIMITING MEANS FOR AN FM/FMMULTIPLEX SIGNAL 2 Sheets-Sheet 1 Dec. 29, 1964 Filed Jan. 24. 1961 Dec. 29, 1964 B. D. I oUGHLIN 3,163,717

SIGNAL-PROCESSING APPARATUS UTILIzING VARIABLE THRESHOLD LIMITING MEANS FOR AN FII/FII MULTIPLEX SIGNAL Filed Jan. 24. 1961 2 Sheets-Sham` 2 TIME FIG. 3

United States Patent O SEGNAL-PRQESlNG APPARATUS UTILIZlNG VARABLE THRESHLD LlMlTlNG MEANS FR AN RTM/FM MULTIPLEX SIGNAL Bernard D. Loughlin, Huntington, NX., assigner to Hazeltiue Research Inc., a corporation of Illinois Filed lan. 24, 1961, Ser. No. 89,S95 8 Claims. (Cl. 179-15) This invention relates to signal-processing apparatus for use in a radio receiver adapted tot receive a frequencymodulation (FM) multiplex signal. More particularly, it relates to an arrangement of the suboarrier channel which makesrthe receiver suitable for stereophonic sound reproduction. l Y

The purpose of stereophonic sound reproduction is to-create in the listener a more realistic illusion of listening to the original sound rather than a reproduction thereof. The idea is that, in addition to reproducing all the frequencies of the sound, it is necessary to give the reproduced sound a further qualitative effect of direction. This is because only in a minority of cases does the original sound emanate from a single point source as happens when a single loudspeaker is used. An example of a multiple source sound is an orchestra with the listener sitting directly in front. Some sounds, for example from the string section, come predominantly from the left side of the orchestra, while others, such as the Woodwinds, come from the right. In recording and reproducing this music stereophonical-ly in order to retain the original t directional effect, two separate signal channels are used, each having its own microphone and loudspeaker. One microphone is placed in front of the left side of the orchestra and the other in front of the right. The signals from the two microphones ,are ultimately applied individually to each of the loudspeakers which are, of course, oriented respectively to the left and right of the listener. Thus, the directional elfect of the music as it was originally produced is maintained to a large extent in the reproduced music, thereby considerably enhancing the listening enjoyment as compared with monaural sound reproduction, i.e., from a single loudspeaker.

It should be `obvious that if the sounds from the left and right sides of the orchestra are of equal loudness, then they should be reproduced with equal loudness in the respective loudspeakers. However, due to possible relative differences in signalprocessing in the two signaltranslating channels and relative differences in loudspeaker eliiciencies, the reproduced soundl might not be Vequally loud in both loudspeakers. For this reason?, a special control is lgenerally provided to enable the relative loudness of the channels to be adjusted until` a balance between the two is achieved, p e

Heretofore, the most common source of stereophonic sound for home reproduction purposes has been the stereophonic record. However, stereophonic radio broadcasting has also been provenfeasible and, as a result, there are presently several proposals before the Federal Communications Commission for it to consider in the adopttion of a standard system of stereophonic broadcasting. Several of the proposed systems utilize an FM/FM multiplex signal in which vthe transmitted signal comprises a I main carrier wave signal, frequency-modulated by one of the audio-frequency stereophonic signals and by a subcarrier of, Vfor example, SO'kilocycles, which, itself, has` been frequency-modulated bythe other audio-frequency stereophonic signal. At the stereophonic FM receiver, the signal at the output of the first, or mainchannel, frequency detector consists of the first stereophonic signal rataudio-frequency'plus the frequency-modu-l lated subcarrier signal at the assumed frequency oi' 50 kilocycles. The subcarrier signal is then separated from the lfirst stereophonic signal and passed through a second, or subcarrier-channel, frequency detector to recover the second stereophonic signal from the subcarrier. In order to ensure faithful stereophonic sound reproduction, the first and second detected stereophonic signals must be maintained at correct amplitude relative to each other; The actual eifect to the listener of any deviation from this requirement will dependY upon the type of stereophonic signals used in the system. For example, if a noncompatible signal is used where one of the signals, either left or right, is modulated on the main carrier `and the other is modulated on the subcarrier, the effect to the listener would be an imbalance in the reproduced sound; that is, the sound from one of the loudspeakers would be louder than that from the other. On the otherhand, if a compatible signal is used Where lat least portions of both the left and right signals are modulated on both carriers and subsequent matrixing is used to separate the left and right signals for application to corresponding left and right loudspeakers, the effect to the listener would be a loss of the directional efect, that is, a loss in the separation of the two signals.

At the present time, it is known practice in an FM receiver to utilize a ratio detector circuit as the usual frequency detector and to rely on the operation of its lautomatic-gain-control (AGC) circuit to provide amplitude rejection. The main reason for this arrangement is economy of construction since, among other things,

"its circuits are not of .as critical a design as a fixed threshold limiter and a discriminator circuit. However, this arrangement is not completely insensitive to .the amplitude of the signal since, as the average' strength of the signal changes, the amplitude of the ratio detector output will vary to some extent. Heretofore it has been common practice in FM/FMV multiplex receivers to Vuse fixed threshold amplitude limiters in the subcarrier channel. When used with the above-described ratio detector arrangement, it can cause improper stereophonic reproduc tion since, as the receiver is tuned from strong to Weak stations, the output of fthe ratio detector will vary, With corresponding variations in the amplitude ofthe main stereophonic signal While the subcarn'er stereophonic signal remains essentially constant due to the lixed threshold limiter. The same effect can occur when tuned to a single station if the received signal should fadeV for any reason.`

It is, therefore, an object of thek present invention to: provide signal-processing apparatus for an FM multiplex channel.

In accordance with the invention', signal-processing Vapparatus for a radio receiver of the type adapted to receive a frequency-modulation multiplex signal having a-main carrier wave signal frequency-modulated by'a lfirst information signal and by a subcarrierV wave"l signal which isY frequency-modulated by` a second information signal comprises means including a first frequency detector re- Sponsive to the multiplex signal forsupplying atan output thereof a composite signal comprising the first information signal and the modulated subcarrier-signal, Vthe.v

composite signal being subject torvariations in its average amplitude. The apparatus further comprises signaltranslating means including a second frequency detector for detecting the second information signal from the subcarrier signal and for maintaining the average amplitude of the second information signal for a given percentage modulation of the subcarrier signal proportional to the strength of the modulated subcarrier signal, whereby variations in the average amplitude of the first information signal resulting from the aforementioned variations in the composite signal are followed by corresponding variations in the average amplitude of the second information signal.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawings:

FIG. 1 is a diagram of a radio receiver constructed in accordance with theteachings of .the present invention to receive an FM/ FM multiplex signal;

FIG. 2 is a circuit diagram showing a particular form of a circuit'for use in the subcarrier channel of the receiver of FlG. 1, and

FGS. 3a-3c, inclusive, are diagrams showing the form of the translated signal at various points in the circuit of FIG. 2.

FM/FM Multiplex Receiver of FIG. 1

The radio receiver shown in FIG. 1 includes an antenna system 1t), coupled to the input of receiver apparatus 11. Apparatus 11 may be conventionally constructed to include the usual radio-frequency amplifier, frequency converter, and intermediate-frequency amplitier circuits operative to supply an FM/FM multiplex signal at an intermediate-frequency of, for example, 10.7 megacycles. In accordance with one of the aforementioned proposed systems, this signal may comprise a main carrier Wave signal at the 10.7 megacycle frequency, frequency-modulated by a first information signal corresponding to the first stereophonic signal and by a subcarrier wave signal at 50 kilocyclcs frequency-modulated by a second information signal with a maximum frequency deviation of i25 kilocycles. At least one proposed form of FM/FM multiplex signal for stereophonic broadcasting.contemplates the use of a first stereophonic signal matrixed at the transmitter to be the sum L-l-R of the left and right sides or channels of the original sound source; while the second stereophonic signal, modulated on the subcarrier, is the difference L-R of the two channels. This is to enable a conventional frequencymodulation receiver to operate compatibly with the stereophonic signalA by simply detecting the sum signal L-t-R, thereby reproducing the complete monaural signal. The following description will assume the reception of such a signal, although it will be appreciated thatgdifferently matrixed signals may be employed without any change in the present invention.

The signal-processing apparatus' ofthe invention includes a first, or main-channel, frequency detector 12 responsive to thevmultiplex signal at the output of receiver apparatus 11 to detect from the 1F carrier and supply at output terminal 13 a composite signal including both the first informationsignal and the modulated subcarrier wave signal.- The first information signal in this instance is the matrixed audio-frequency stereophonic signal L-t-R. Although frequency detector 12 may be of any conventional type as, for example, a fre-V quency discriminatorgyit is anticipated that the present invention will be most useful ina case Where detector 1'2 isaratio detector, additionally deriving an automatic-v gain-control bias which is fed back to receiver apparatus 11 to control the gain of the amplifier circuits therein. As previously mentioned with this arrangement, itV is possible, especially in weak signal areas, for the composite signal at terminal 13 to be subject to undesirable average amplitude variations capable of producing a noticeable degradation in the stereophonic effect of the reproduced sound.

The composite signal at terminal 13 is coupled to low-pass filter 14, wherein the stereophonic signal L-i-R is selected and passed through to one input of matrix amplifier 15. The modulated subcarrier is prevented from being translated to matrix amplifier 15 by virtue of the pass band characteristic of lter 14 which should have an upper limit that is effectively less than the 10W- est frequency component of the modulated subcarrier.

Since the subcarrier signal is at a high frequency rela- 15 tive to the audio frequencies of the first stereophonic signal, there should be no de-emphasis, i.e., attenuation of high-frequency components of the signal prior to terminal 13. Thus, filter 14 may simply be the usual rie-emphasis circuit inherently blocking translation of any signal component above the audio-frequency range.

The composite signal at terminal 13 is also applied to a signal-translating circuit, hereinafter referred to as a subcarrier channel, for detecting the second stereophonic signal L-R from the subcarrier wave signal. Specifically,

the signal is applied to the input of bandpass filter 16, preferably having a pass Vband characteristic centered about the subcarrier frequency and a bandwidth which will pass all the frequency components of the modulated subcarrier signal, while preventing the translation of any substantial amount of the higher audio-frequency components of the first signal L-t-R. This is to assure that undesired crosstalk between the main and subcarrier channels caused by the direct leakage of the signal L-i-R into Q the subcarrier channel is minimized.

"D The modulated subcarrier at the output of filter 16 is then applied, in accordance with the teachings of the present invention, to a frequency detector arrangement that preferably detects the signal L-R and maintains the level thereof, for a given percentage modulation value, proportional to the strength of the modulated subcarrier wave signal. Specifically, the modulated subcarrier wave signal is applied to variable threshold limiter circuit 17 which, by virtue of the operation of filter 16, is responsive a substantially only to the supplied modulated subcarrier to 4D produce therefrom an amplitude-limited signal having a peak-to-peak value that is proportional to the strength of the modulated subcarrier. The amplitude-limited signal is then applied to the input of a second frequency detector 13 wherein the second information signal L-R is detected from the subcarrier. Detector 18 ,includes a de-emphasis circuit which may be of the same type as that used in filter 14. Frequency detector 18 may be of the conventional pulse-counter type or of the improved pulse-counter type described in connection with FIG. 2.

It is well known that a pulse-counter frequency detector requires some form of amplitude limiting of the input signal thereto, and it is the purpose of variable threshold limiter circuit 1'7 to perform such a function while, at the same time, adapting the subcarrier channel to maintain 00 the level of the detected signal L-R, for a given percentage modulation of the subcarrier, proportional to the strength of the modulated subcarrier signal. With the arrangement just described, undesired variations in the amplitude of the composite signal at terminal 13, causing 6i variations in the amplitude of signal L-l-R, are automatically followed by corresponding variations in the average amplitude of the subcarrier signal Lx-R.

The output of frequency detector 18, i.e., audio-frequency signal L-R, is coupled to a second input of matrix amplifier'lS, wherein appropriate addition and subtraction operations are performed to provide at first and second outputs thereof the signals R and L, respectively coupled to speakers SR and SL for stereophonic sound reproduction.

To more fully appreciate the advantages of this inven- 75 tion, it will b e helpful 'to consider, in a little more detail,

the natureof the operations occurring in matrix amplifier 15. In one half of amplifier the signals L-i-R and L-R are added to produce the signal 2L at terminal 15a, while in theother half thereof the signal L-R is effectively subtracted from the signal L-{R to produce the signal 2R at terminal 15b. If, however, either of the input ignals L-t-R and L-R is changed in amplitude relative to the other, aV reduction in the stereophonic efiectof the reproduced sound will occur caused by some of the signal R appearing in the left speaker SL and vice versa. For example, assume initially that, in accordance with present practice, a fixed threshold limiter is used in the subcarrier channel of FIG. 1, andthat the level of the signals L-i-R and L-R at the input of matrix amplifier 15 are both equal. This is the situation previously described where the signals 2L and 2R appear individually at terminals 15a and 15b, respectively, and this, of course, is the desired manner of operation. However, if now the composite signal `at terminal 13 increases to twice the initial amplitude, the signal from the main channel filter M will, naturally, be increased in amplitude to'2L-l-2R, while the signal from the frequency detector 18 will remain unchanged in amplitude at Lf-R due to the operation of the fixed threshold limiter. The addition and subtraction operationsin matrix amplifier 15 will then produce at terminals 15a and 15b the signals SL-l-R and 3R-l-L, respectively. There is thus some undesired signal feedthrough from each channel into the other. On the other hand, if, instead of a fixed threshold limiter, the variable vthreshold limiter is used in the subcarrier channel in accordance with this invention, a subcarrier output would also double, giving the signal 2L-2R which, at the output of the matrix, would produce balanced signals 4L and 4R. Thus proper stereophonic operation is assured whenever variations in the level of the main channel signal L-[ R result from variations `in the signal' level of the composite signal at terminal 13. This 1automatic trackingis achieved because of the fact that the level of the signal L-R is maintained proportional to the strength of the modulated subcarrier signal at terminal 13, the strength of the subcarrier being used as an indication of the amount of variations inV the composite signal.

It will be appreciated that, conventionally, the level of the detected signal L-R is proportional to the percentage modulation of the subcarrier signal. In other Words, a high amplitude signal results from a high percentage modulation of the subcarrier and a low amplitude signal results from a low percentage modulation. However, for purposes of describing the present invention, the phrase variations in the level (or average amplitude) of the signal L-R is here intended to mean those variations produced by variations in` the strength of the modulated subcarrier and not those variations produced at the transmitter by different percentage modulations thereof.

Subcarrer Channel of FIG. 2

In FIG. 2 the subcarrierchannel ofthe receiver of FIG. 1 is shown with a specific circuit arrangement for use in units'l and 18. The composite signal supplied from frequency detector 12 to terminal 13 is applied through bandpass filter 16 to block the audio-frequency signal L--R, as previously mentioned, and to provide at the output thereof only the modulated subcarrier components. Signal A of FIG. 3a represents the subcarrier signal in unmodulated form as it appears at the input of filter 16. Filter 16 may include a stage of amplification at the output thereof operating to invert signal A which is then applied through capacitor 20 to one end terminal 21 of variable threshold limiter circuit 17. Both peaks of the amplified and inverted signal are clipped symmetrically in limiter 17 to form signal B of FIG. 3b. Limiter circuit17 includes diode 22,k poled conductively from terminal 21 to a point of fixed potential, such as ground, and further includes diode 23, poled conductively from ground through resistor 24 back to terminal 21. As `will be seen sub- 6 sequently, the clipping level, that is, the peak-to-peak level of the subcarrier signal, remains a constant percentage of the unclipped signal level, regardless of any Variations in amplitude of the unclipped signal, with the result that the amplitude of the limited signal is maintaineddirectly proportional to the amplitude of the subcarrier signal A.

The actual clipping level is determined by the value of resistor 24, which value is selected to be a compromise between a higher amplitude output signal with a higher value resistance and better downward AM handling capability with a lower value resistance. A bypass capacitor 25 is coupled across resistor 24 to hold the clipping level relatively constant in the presence of incidental amplitude modulation of the subcarrier at audio frequency, such as noise or crosstalk. The criteria for selecting the proper value for capacitor 25 will be considered in connection with the description of auxiliary balancing detector circuit 33.

The amplitude-limited signal B is applied to a pulsecounter type of frequency detector. Specifically, signal B at terminal 21 is applied to a differentiator circuit including capacitor 26 and resistor 27 having a time constant which is short relative to the period of ahalf cycle of signal B. Diode 28, conductively coupled from capacitor 26 to ground, is provided to clip off the positive-going pulses produced by differentiator circuit 26, 27 and to clamp the base line of the remaining negative pulses at ground potential, thereby forming signal C in FIG. 3c. An inductor 29 is connected in series with dierentiator circuit 26, 27 to simultaneously provide better linearity and increased output as compared to operation of pulsecounter detector 18 with merely a simple differentiator circuit. This feature of detector 18 is the subject matter of applicants copending application Serial No. 84,636, filed January 24, 1961. A de-emphasis circuit, comprising resistor 3ft and capacitor 31, is connected across the output of detector 18 to compensate in the usual manner for the high-frequency pre-emphasis inserted at the transmitter. Circuit 30, 31 also serves to filter out' the subcarrier frequency pulses from the output of detector 18, thereby leaving, at terminal 32, only the average amplitude component of the pulses, which, for an unmodulated subcarrier, would be a` D.C. voltage. It will be appreciated that withconstant area pulses the average amplitude component of signal C will vary only as the number of pulses in signal C varies over a given period of time, thus producing an output' signal representative of the frequency modulation of the subcarrier.

In certain instances, it can be shown that the frequencymodulated subcarrier signal, as it appears at the output of frequency detector 12, may also be amplitude-modulated to some degree by the first information signal. If this amplitude-modulation is not suppressed in the subcarrier channel, undesired signals will result at terminal 32, due tot he operation of diode 28 as an amplitude detector. Limiter circuit 17 may not operate to completely suppress this amplitude-modulation and, therefore, to improve the amplitude-modulation rejection in the subcarrier channel, an auxiliary balancing circuit is coupled across detector 1,8 to detect the amplitude-modulation out of phase with the detected amplitude-modulation in the detector 18 and add it back into the output of Vdetector 1S to cancel out the resulting undesired signals.

This auxiliary balancingl circuit comprises an amplitude detector 33 to which is applied the amplitude-limited signal B (which may not be perfectly limited) from terminal 21 through capacitor 34. Amplitude detector 33 includes diodes 35 and 36, across which is connected the usual filter arrangement of capacitor `3'7 and resistor 38. The detected signal is then applied to output terminal 32 by adjustable resistor 39. Diodes 2x5-and 36 are poled to clarnp the negative peaks of signal B to groundand to charge capacitor 37 to the positive peaks thereof. It will be noted that frequency detector 1.8 produces a negative polarity output signal from the pulses of signal C and that amplitude detector 33 produces a positive polarity output in response to output variations of signal B. Thus, an increase in amplitude of signal B produces an increase in the negative direction at the output of detector 18 and an increase in the positive direction at the output of detector 33, both of which cancel each other out at terminal 32, leaving only the stereophonic signal L-R derived from the frequency demodulation of the subcarrier signal. It will be appreciated that the time delay occurring in detector 1S and detector 33 must be substantially the same in order to maintain the 180 phase relation for proper signal cancellation. Resistor 39 is included to provide an adjustable control of the amount of amplitude-modulaiton cancellation, thereby permitting optimization of the amplitude-rejection characteristic of signal-translating circuit 18. The foregoing balanced circuit feature of the subcarrier channel circuit is the subject of applicants copending application Serial No. 84,667, iiled January 24, 1961, now U.S. Patent No. 3,128,437.

Referring again to FIGS. 3a3c, there is shown therein the effect that a change in the average amplitude of the applied subcarrier signal has on the operation of the circuit of FIG. 2. If signal A decreases to signal A', the limited signal at terminal 22 decreases to B', producing a smaller amplitude for a series of pulses in signal C', thereby resulting in a decrease in the amplitude of the average voltage at output terminal 32. Correspondingly, an increase in signal A to A" produces a higher amplitudelimited signal B. Correspondingly an increase in signal A to A increases the limited signal B", producing the higher amplitude pulses in signal C", causing an increase in the average amplitude occurring at terminal 32. The result is an automatic tracking by signal L-R of the variations in the main signal L-j-R produced by variations in the average amplitude of the composite signal at' terminal 13.

While applicant does not Wish to be limited to any particular set of circuit constants, the following have proved useful in the signal-processing apparatus of the present invention.

Resistor 24 27 kilohms.

Resistor 27 33 kilohms.

Resistor 30 100 kilohms.

Resistor 38 330 ltilohms.

Resistor 39 1 megohm-variable Capacitor 20 0.01 microfarad. Capacitor 25 8 microfarads. Capacitor 26 100 micromicrofarads. Capacitor 31 1500 micromicrofarads. Capacitor 34 470 micromicrofarads. Capacitor 37 220 micromicrofarads. Inductor 29 50 millihenries. Diodes 22, 23, 28, 35, 36 1N34AS.

wWhile there has been described what' is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

What is claimed is:

1. Signal-processing apparatus for a radio receiver adapted to receive a frequency-modulation multiplex signal having a main carrier wave signal frequency-modulated by a first information signal and by a subcarrier Wave signal which is frequency-modulated by a second information signal, said apparatus comprising: means including a first frequency detector responsive to said multiplex signal for supplying a composite signal comprising said first information signal and said modulated subcarrier signal, said composite signal being subject to variations in its average amplitude; and signal-translating means including a second frequency detector for detecting said second information signal from the subcarrier and means for maintaining the average amplitude of the second information signal, for a given percentage modulation of the subcarrier, proportional to the strength of the subcarrier; whereby variations in the average amplitude of the first information signal, resulting from said variations in the composite signal, are followed by corresponding variations in the average amplitude of the second information signal.

2. Signal-processing apparatus for a radio receiver adapted to receive a frequency-modulation multiplex signal having a main carrier Wave signal frequency-modulated by a first information signal and by a subcarrier wave signal which is frequency-modulated by a second information signal, said apparatus comprising: means including a iirst frequency detector responsive to said multiplex signal for supplying a composite signal comprising said first information signal and said modulated subcarrier signal, said composite signal being subject to variations in its average amplitude; and signal-translating means including a second frequency detector for detecting said second information signal from the subcarrier and variable threshold limiter circuit means for maintaining the average amplitude of the second information signal, for a given percentage modulation of the subcarrier, proportional to the strength of the subcarrier; whereby variations in the average amplitude of the rst information signal, resulting from said variations in the composite signal, are followed by corresponding variations in the average amplitude of the second information signal.

3. Signal-processing apparatus for a radio receiver adapted to receive a frequency-modulation multiplex signal having a main carrier wave signal frequency-modulated by a first information signal and by a subcarrier wave signal which is frequency-modulated by a second information signal, said apparatus comprising: means including a iirst frequency detector responsive to said multiplex signal for supplying a composite signal comprising said first information signal and said modulated subcarrier signal, said composite signal being subject to variations in its average amplitude; and signal-translating means including a second frequency detector of the pulse-counter type for detecting said second information signal from the subcarrier and means for maintaining the average amplitude of the second information signal, for a given percentage modulation of the subcarrier, proportional to the strength of the subcarrier; whereby variations in the average amplitude of the iirst information signal, resulting from said variations in the composite signal, are followed by corresponding variations in the average ampltiude of the second information signal.

4. Signal-processing apparatus for a radio receiver adapted to receive a frequency-modulation multiplex signal having a main carrier wave signal frequency-modulated by a first information signal and by a subcarrier wave signal which is frequency-modulated by a second information signal, said apparatus comprising: means including a first frequency detector responsive to said multiplex signal for supplying a composite signal comprising said rst information signal and said modulated subcarrier signal, said composite signal being subject to variations in its average amplitude; and signal-translating means including a second frequency detector of the pulsecounter type for detecting said second information signal from the subcarrier and variable threshold limiter circuit means for maintaining the average amplitude of the second information signal, for a given percentage modulation of the subcarrier, proportional to the strength of the subcarrier; whereby variations in the average amplitude of the first information signal, resulting from said variations in the composite signal, are followed by corresponding variations in the average amplitude of the second information signal.

5. Signal-processing apparatus for a stereophonic radio receiver adapted to receive a frequency-modulation multiplex signal having a main carrier wave signal frequencymodulated by a first stereophonic signal and by a subcarrier wave signal which is frequency-modulated by a second stereophonic signal, said apparatus comprising: means including a first frequency detector responsive to said multiplex signal for supplying a composite signal coinprising said rst stereophonic signal and saidV modulated subcarrier signal, said composite signal being subject to variations in its average amplitude; and signal-translating means including a second frequency detector for detecting said second stereophonic signal from the subcarrier and means for maintaining the average amplitude of the second stereoplionic signal, for a given percentage modulation of the subcarrier, proportional to the strength of the subcarrier; whereby variations in the average amplitude of the first stereophonic signal, resulting from said Variations in the composite signal, are followed by corresponding variations in the average amplitude of the second stereophonic signal.

6. Signal-processing apparatus for a radio receiver adapted to receive a frequency-modulation multiplex signal having a main carrier Wave signal frequency-modulated by a first information signal and by a subcarrier wave signal which is frequency-modulated by a second information signal, said apparatus comprising: means including a first frequency detector of the ratio detector type responsive to said multiplex signal for supplying a composite signal comprising said first information signal and said `modulated subcarrier signal, said composite signal being subject to Variations in its average amplitude; and signaltranslating means including a second frequency detector for detecting said second information signal from the subcarrier and means for maintaining the average amplitude of the second information signal, for a given percentage modulation of the subcarrier, proportional to the strength f the subcarrier; whereby variations in the average amplitude of the rst information signal, resulting from said variations in the composite signal, are followed by corresponding variations in the average amplitude of the second information signal.

7. Signal-processing apparatus for a stereophonic radio receiver adapted to receive a frequency-modulation multipleX signal having a main carrier wave signal frequencymodulated by a first stereophonic signal and by a subcarrier wave signal which is frequency-modulated by a second stereophonic signal, said apparatus comprising: means including a first frequency detector of the ratio detector type responsive to said multiplex signal for supplying a composite signal comprising said first stereophonic signal and said modulated subcarrier signal, said composite signal being subject to variations in its average amplitude; and signal-translating means including a second frequency detector for detecting said second stereophonic signal from the subcarrier and means for maintaining the average amplitude of the second stereophonic signal, for a given percentage modulation of the subcarrier, proportional to the strength of the subcarrier; whereby variations in the average amplitude of the first stereophonic signal, resulting from said variations in the composite signal, are followed by corresponding variations in the average amplitude of the second stereophonic signal.

8. Signal-processing apparatus for a stereophonic radio receiver adapted to receive a frequency-modulation multiplex signal having a main carrier Wave signal frequencymodulated by a first stereophonic signal and by a subcarrier wave signal which is frequency-modulated yby a second stereophonic signal, said apparatus comprising: means including a first frequency detector of the ratio detector type responsive to said multiplex signal for supplying a composite signal comprising said first stereophonic signal and said modulated subcarrier signal, said composite signal being subject to variations in its average amplitude; and signal-translating means including a second frequency detector of the pulse-counter type for detecting said second stereophonic signal from the subcarrier and variable threshold limiter circuit means for maintaining the average amplitude of the second stereophonic signal, for a given percentage modulation of the subcarrier, proportional to the strength of the subcarrier; whereby variations in the average amplitude of the first stereophonic signal, resulting from said variations in the composite signal, are followed by corresponding variations in the average amplitude of the second stereophonic signal.

References Cited in the file of this patent UNlTED STATES PATENTS 2,851,532 Crosby Sept. 9, 1958 3,059,056 Freedman et al. Oct. 16, 1962 3,059,189 Preisg Oct. 16, 1962 

1. SIGNAL-PROCESSING APPARATUS FOR A RADIO RECEIVER ADAPTED TO RECEIVE A FREQUENCY-MODULATION MULTIPLEX SIGNAL HAVING A MAIN CARRIER WAVE SIGNAL FREQUENCY-MODULATED BY A FIRST INFORMATION SIGNAL AND BY A SUBCARRIER WAVE SIGNAL WHICH IS FREQUENCY-MODULATED BY A SECOND INFORMATION SIGNAL, SAID APPARATUS COMPRISING: MEANS INCLUDING A FIRST FREQUENCY DETECTOR RESPONSIVE TO SAID MULTIPLEX SIGNAL FOR SUPPLYING A COMPOSITE SIGNAL COMPRISING SAID FIRST INFORMATION SIGNAL AND SAID MODULATED SUBCARRIER SIGNAL, SAID COMPOSITE SIGNAL BEING SUBJECT TO VARIATIONS IN ITS AVERAGE AMPLITUDE; AND SIGNAL-TRANSLATING MEANS INCLUDING A SECOND FREQUENCY DETECTOR FOR DETECTING SAID SECOND INFORMATION SIGNAL FROM THE SUBCARRIER AND MEANS FOR MAINTAINING THE AVERAGE AMPLITUDE OF THE SECOND INFORMATION SIGNAL, FOR A GIVEN PERCENTAGE MODULATION OF THE SUBCARRIER, PROPORTIONAL TO THE STRENGTH OF THE SUBCARRIER; WHEREBY VARIATIONS IN THE AVERAGE AMPLITUDE OF THE FIRST INFORMATION SIGNAL, RESULTING FROM SAID VARIATIONS IN THE COMPOSITE SIGNAL, ARE FOLLOWED BY CORRESPONDING VARIATIONS IN THE AVERAGE AMPLITUDE OF THE SECOND INFORMATION SIGNAL. 